fix: add 1px inset border between macOS window frame and app content

The transparent titlebar window blended with the app content because
both shared the same background color. An inset box-shadow using the
existing --border-primary color creates a subtle separation. The 10px
border-radius matches the macOS window corner radius.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/App.css

@@ -5,6 +5,8 @@
   height: 100vh;
   width: 100vw;
   overflow: hidden;
+  border-radius: 10px;
+  box-shadow: inset 0 0 0 1px var(--border-primary);
 }
 
 .app {